Arenediazonium salt reaction with

Arenediazonium salts reacted with tetramethyltin under very mild conditions in acetonitrile yielding the corresponding toluenes [63] and this reaction could be carried out in aqueous media, as well [64] (Scheme 6.29). Similar to the Heck reactions discussed in 6.1.1, a one-pot procedure could be devised starting from anilines, with no need for the isolation of the intermediate diazonium salts. The pH of the solutions should always be kept below 7 in order to avoid side reactions of the diazonium salts, however, unlike with the Heck reactions, HCl or H2SO4 can also be used. Since organotin compounds are easily hydrolysed in acidic solutions, a careful choice of the actual pH is required to ensure fast and clean reactions. Diaryliodonium salts are hydrolytically stable and also react smoothly with various organotin compounds (Scheme 6.29) [65]. 

Sengupta showed that the reaction of bis-arenediazonium salt 91 with vinyl(triethoxy)silane 92 afforded poly(phenylene-vinylene) 93. Although the reaction apparently proceeds through the Heck reaction mechanism, which is described in Section 11.19.4, a part of the step-growth reaction is indeed a transformation of the carbon-silicon bond of 92 to the carbon-carbon bond (Equation (44)). 

This diazotization reaction is compatible with the presence of a wide variety of substituents on the benzene ring. Arenediazonium salts are extremely important in synthetic chemistry, because the diazonio group (N=N) can be replaced by a nucleophile in a radical substitution reaction, e.g. preparation of phenol, chlorobenzene and bromobenzene. Under proper conditions, arenediazonium salts react with certain aromatic compounds to yield products of the general formula Ar-N=N-Ar, called azo compounds. In this coupling reaction, the nitrogen of the diazonium group is retained in the product. 

Arenediazonium salts react with cuprous chloride, cuprous bromide, and cuprous cyanide to give products in which the diazonium group has been replaced by —Cl, —Br, and — CN, respectively. These reactions are known generally as Sandmeyer reactions. Several specific examples follow. The mechanisms of these replacement reacaiions are not fully understood the reactions appear to be radical in nature, not ionic. 

Arenediazonium salts are also used for the couplina[563], (Z)-Stilbene was obtained unexpectedly by the reaction of the ti-stannylstyrene 694 by addition-elimination. This is a good preparative method for cu-stilbene[564]. The rather inactive aryl chloride 695 can be used for coupling with organostannanes by the coordination of Cr(CO)3 on aromatic rings[3.565]. 

Reaction with arenediazonium salts Adding a phe nol to a solution of a diazonium salt formed from a primary aromatic amine leads to formation of an azo compound The reaction is carried out at a pH such that a significant portion of the phenol is pres ent as its phenoxide ion The diazonium ion acts as an electrophile toward the strongly activated ring of the phenoxide ion... 

Arylhydrazones from reaction of /3-dicarbonyl compounds with arenediazonium salts... 

The reaction mechanism is not rigorously known, but is likely to involve the following steps." " First the arenediazonium ion species 1 is reduced by a reaction with copper-(l) salt 2 to give an aryl radical species 4. In a second step the aryl radical abstracts a halogen atom from the CuXa compound 5, which is thus reduced to the copper-1 salt 2. Since the copper-(l) species is regenerated in the second step, it serves as a catalyst in the overall process. 

For the in situ preparation of the required arenediazonium salt from an aryl amine by application of the diazotization reaction, an acid HX is used, that corresponds to the halo substituent X to be introduced onto the aromatic ring. Otherwise—e.g. when using HCl/CuBr—a mixture of aryl chloride and aryl bromide will be obtained. The copper-(l) salt 2 (chloride or bromide) is usually prepared by dissolving the appropriate sodium halide in an aqueous solution of copper-(ll) sulfate and then adding sodium hydrogensulfite to reduce copper-(ll) to copper-(1). Copper-(l) cyanide CuCN can be obtained by treatment of copper-(l) chloride with sodium cyanide. 

Aryl chlorides and bromides are prepared by reaction of an arenediazonium salt with the corresponding copper(I) halide, CuX, a process called the Sandmeyer reaction. Aryl iodides can be prepared by direct reaction with Nal without using a copper(T) salt. Yields generally fall between 60 and 80%. 

Similar treatment of an arenediazonium salt with CuCN yields the nitrile, ArCN, which can then be further converted into other functional groups such as carboxyl, for example, Sandmeyer reaction of o-methylbenzenediazonium bisulfate with CuCN yields o-methylbenzonitrile, which can be hydrolyzed to give o-methylbenzoic acid. This product can t be prepared from o-xvlene by the usual side-chain oxidation route because both methyl groups would be oxidized. 

The diazonio group can also be replaced by —OH to yield a phenol and by —H to yield an arene. A phenol is prepared by reaction of the arenediazonium salt with copper(I) oxide in an aqueous solution of copper(ll) nitrate, a reaction that is especially useful because few other general methods exist for introducing an -OH group onto an aromatic ring. 

Arenediazonium salts undergo a coupling reaction with activated aromatic rings such as phenols and arylamines to yield brightly colored azo compounds, Ar—N=N—Ar. ... 

Arylamines are converted by diazotization with nitrous acid into arenediazonium salts, ArN2+ X-. The diazonio group can then be replaced by many other substituents in the Sandmeyer reaction to give a wide variety of substituted aromatic compounds. Aryl chlorides, bromides, iodides, and nitriles can be prepared from arenediazonium salts, as can arenes and phenols. In addition to their reactivity toward substitution reactions, diazonium salts undergo coupling with phenols and arylamines to give brightly colored azo dyes. 

Sandmeyer reaction (Section 24.8) The nucleophilic substitution reaction of an arenediazonium salt with a cuprous halide to yield an aryl halide. 

Another redox reaction leading to arenediazonium salts was described by Morkov-nik et al. (1988). They showed that the perchlorates of the cation-radicals of 4-A,A-dimethylamino- and 4-morpholinoaniline (2.63) react with gaseous nitric oxide in acetone in a closed vessel. The characteristic red coloration of these cation-radical salts (Michaelis and Granick, 1943) disappears within 20 min., and after addition of ether the diazonium perchlorate is obtained in 84% and 92% yields, respectively. This reaction (Scheme 2-39) is important in the context of the mechanism of diazotization by the classical method (see Sec. 3.1). 

When an aqueous solution of an arenediazonium salt is added to an alkaline buffer solution, an initial rapid reaction occurs. All experimental evidence is consistent with the hypothesis that only the (Z)-diazoate is formed. Theoretically, however, the competitive formation of the (ii)-isomer in very small quantities cannot be excluded... 

The P-coupling of 1,2- and 1,4-quinonediazides with phosphines (mainly tri-phenylphosphine) was studied intensively in the 1960s by the groups of Ried and Horner (see summary by Ershov et al., 1981, p. 147). The azo derivatives formed in these reactions are more stable than those of arenediazonium salts because of the stabilization by mesomeric delocalization (6.35 a ++ 6,35 b). 

The fundamental understanding of the diazonio group in arenediazonium salts, and of its reactivity, electronic structure, and influence on the reactivity of other substituents attached to the arenediazonium system depends mainly on the application of quantitative structure-reactivity relationships to kinetic and equilibrium measurements. These were made with a series of 3- and 4-substituted benzenediazonium salts on the basis of the Hammett equation (Scheme 7-1). We need to discuss the mechanism of addition of a nucleophile to the P-nitrogen atom of an arenediazonium ion, and to answer the question, raised several times in Chapters 5 and 6, why the ratio of (Z)- to ( -additions is so different — from almost 100 1 to 1 100 — depending on the type of nucleophile involved and on the reaction conditions. However, before we do that in Section 7.4, it is necessary to give a short general review of the Hammett equation and to discuss the substituent constants of the diazonio group. 

In this section a general introduction to homolytic dediazoniation of arenediazonium salts is given, with some representative examples. The following sections of this chapter first describe reaction conditions for observing the changeover from hetero-... 

As an alternative to electrochemical or radiolytic initiation, homolytic dediazoniation reaction products can be obtained photolytically. The organic chemistry of such photolyses of arenediazonium salts will be discussed with regard to mechanisms, products, and applications in Section 10.13. In the present section photochemical investigations are only considered from the standpoint that the photolytic generation of aryldiazenyl radicals became the most effective method for investigating the mechanisms of all types of homolytic dediazoniations —thermal and photolytic —in particular for elucidating the structure and the dissociation of the diazenyl radicals. 

Aldehydes and ketones are formed in reactions of arenediazonium salts with derivatives of carbonyl compounds, e.g., with oximes (Scheme 10-40, Jolad and Ra-jagopal, 1973) or with diacetyl (Scheme 10-41, Citterio et al., 1982b). In most cases yields are mediocre. Better results are obtained with CO, tetraalkyltin, and Pd(OAc)2 as catalyst (Kikukawa et al., 1987). 

Carboxamides and esters of arenecarboxylic acids are obtainable directly by reacting arenediazosulfones (Ar — N2 —S02 —Ar ) with CO and amines or alcohols, respectively, in the presence of Pd catalysts (Kamigata et al., 1989). Aromatic aldehydes are formed if the reaction is carried out in the presence of triethylsilane (Kikukawa et al., 1984). In an analogous way, arenediazonium salts can be transformed into ketones (ArCO —R R = CH3, C2H5, or C6H5) in the presence of stan-nanes, R4Sn (Kikukawa et al., 1982). 

The diazonio group of arenediazonium salts can be replaced by alkenes and alkynes or, seen from the other reaction partner, alkenes and alkynes can be arylated with arenediazonium salts. The reactions are catalyzed by copper salts and, as found more recently, also by salts of palladium and other metals. 

More recently, Beckwith demonstrated that intramolecular Meerwein reactions are also possible if one uses an arenediazonium salt with an aliphatic side-chain in the ortho position containing a double or triple CC bond in 8-position. We will discuss them in Section 10.11. 

Meerwein reactions can conveniently be used for syntheses of intermediates which can be cyclized to heterocyclic compounds, if an appropriate heteroatom substituent is present in the 2-position of the aniline derivative used for diazotization. For instance, Raucher and Koolpe (1983) described an elegant method for the synthesis of a variety of substituted indoles via the Meerwein arylation of vinyl acetate, vinyl bromide, or 2-acetoxy-l-alkenes with arenediazonium salts derived from 2-nitroani-line (Scheme 10-46). In the Meerwein reaction one obtains a mixture of the usual arylation/HCl-addition product (10.9) and the carbonyl compound 10.10, i. e., the product of hydrolysis of 10.9. For the subsequent reductive cyclization to the indole (10.11) the mixture of 10.9 and 10.10 can be treated with any of a variety of reducing agents, preferably Fe/HOAc. 

Another arylation reaction which uses arenediazonium salts as reagents and is catalyzed by copper should be discussed in this section on Meerwein reactions. It is the Beech reaction (Scheme 10-49) in which ketoximes such as formaldoxime (10.13, R=H), acetaldoxime (10.13, R=CH3), and other ketoximes with aliphatic residues R are arylated (Beech, 1954). The primary products are arylated oximes (10.14) yielding a-arylated aldehydes (10.15, R=H) or ketones (10.15, R=alkyl). Obviously the C=N group of these oximes reacts like a C = C group in classical Meerwein reactions. It is interesting that the addition of some sodium sulfite is necessary for the Beech reaction (0.1 to 0.2 equivalent of CuS04 and 0.03 equivalent of Na2S03). 

Kochi (1956a, 1956b) and Dickerman et al. (1958, 1959) studied the kinetics of the Meerwein reaction of arenediazonium salts with acrylonitrile, styrene, and other alkenes, based on initial studies on the Sandmeyer reaction. The reactions were found to be first-order in diazonium ion and in cuprous ion. The relative rates of the addition to four alkenes (acrylonitrile, styrene, methyl acrylate, and methyl methacrylate) vary by a factor of only 1.55 (Dickerman et al., 1959). This result indicates that the aryl radical has a low selectivity. The kinetic data are consistent with the mechanism of Schemes 10-52 to 10-56, 10-58 and 10-59. This mechanism was strongly corroborated by Galli s work on the Sandmeyer reaction more than twenty years later (1981-89). 

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